My mother has switched from WhatsApp to Signal Messenger recently, and immediately started complaining that pictures took many hours to send, which was never the case with WhatsApp.
For example, she would send me a picture at 11am, and it would remain "spinning" until the evening, when it would finally send.
After some investigation, it appears that the delay is only over her Sky Broadband ISP connection (which she is mainly using, due to staying in due to the pandemic).
If she disables WiFi, and enables mobile data on her phone, she uses her phone data connection (another provider), and messages send immediately.
The Sky connection does seem to allow the picture messages through after 9pm though. This is what gave the impression of very slow picture message sending.
Sky Broadband Shield
Sky Broadband comes by default with Sky Broadband Shield enabled (a controversial decision).
This says "Automatically switch to the 18 age-rating during hours you choose - like at night when the kids are in bed" which seems to explain the relaxing of the 9pm block.
So as far as I can tell, it seems that Sky's Broadband Shield blocks Signal's picture messages when it is enabled.
It seems Sky blocked WhatsApp in the past, but I presume they had to unblock it due to customer feedback.
I've advised her to ring Sky Broadband and get the filtering disabled, or better still, change to an ISP that doesn't filter connections.